The invention relates to an optical wavefront modifier for modifying a wavefront of an optical beam passing through the modifier, the modifier comprising a first and a second transparent electrode layer and a medium for modifying the wavefront in dependence on electrical excitation of the medium and arranged between the electrode layers, the beam having a cross-section in the plane of the medium. The invention also relates to a device for scanning an optical record carrier having an information layer.
An optical wavefront modifier is used to change the shape of the wavefront of a radiation beam by introducing path length differences in dependence on the position in the cross-section of the beam. It may be used to change properties of an optical beam such as its vergence by introducing a focus curvature in the wavefront of the beam or to change the direction of the beam by introducing tilt. A wavefront modifier may also operate as a wavefront compensator for compensating an undesired shape of the wavefront of an optical beam, e.g. for removing spherical aberration or coma from a wavefront.
European Patent Application No. 0 745 980 shows an optical scanning device provided with a wavefront modifier used as a tilt compensator. The wavefront compensator is an electrostriction device arranged in the optical path between the radiation source and the objective system. It comprises two electrode layers on each side of an electrostrictive medium. One of the electrode layers comprises three transparent electrodes, each of which covers part of the cross-section of the beam in the plane of the medium. The other electrode layer consists of single transparent electrode covering the entire cross-section of the beam. The wavefront modifier is used to introduce coma in the radiation beam in order to compensate the coma caused by tilt of the record carrier being scanned by the optical scanning device. It is a disadvantage of the known wavefront modifier the aberration compensation does not operate properly when the radiation beam is following a track on the record carrier.
It is an object of the invention to provide a wavefront modifier which provides a good aberration compensation independent of the tracking of the focus.
This object is achieved if, according to the invention, the electrode layers of the wavefront modifier are adapted to impress a first wavefront modification of a first order of a radius in the cross-section and simultaneously a second wavefront modification of a second order of the radius different from the first order. The invention is based on the insight that a first wavefront modification, which is centred on the optical axis of the radiation beam causes other wavefront modifications in the radiation beam when the objective system is displaced from its centred position in a transverse direction when following a track. The other modifications have a radial order different from that of the first modification. In general, the introduction of a first modification in a displaced radiation beam requires the introduction of the first modification and other modifications of different radial order in the not-displaced radiation beam. The wavefront modifier according to the invention introduces the first modification and at least one other modification in the radiation beam. The wavefront modifications are centred on the axis of the radiation beam, unless otherwise stated.
The mathematical function describing spherical aberration has a radial order of four, coma of three, astigmatism and defocus of two and wavefront tilt of one.
The wavefront modifications may be introduced as a decentred first modification or as a combination of a centred first modification and a centred second modification. When introduced as a first and a second modification, the first electrode layer preferably comprises an electrode configuration for effecting the first wavefront modification and the second electrode layer comprises an electrode configuration for independently effecting the second wavefront modification. Alternatively, the electrode configuration of one electrode layer may be adapted to effect independently both the first and second modification. The independent control of the first and second modification allows a compensation of a variable amount of displacement of the radiation beam.
When the wavefront modification is introduced as a decentred modification, the electrode configuration of one electrode layer may be adapted to effect the modification. The amount of decentring of the configuration is preferably substantially equal to the displacement of the radiation beam, i.e. equal to the displacement of the objective system. The electrode configuration may be relatively simple. An off-centre modification may be described as a linear combination of a first modification of the same type as the off-centre modification but centred on the optical axis and a centred second modification of a lower radial order than the first modification. This can be explained as follows. The second modification is the difference between the off-centre modification and the same centred modification. For a small decentring, the second modification is proportional to the derivative of the modification in the radial direction of the decentring. In this case the radial order of the second modification is at least one order lower than that of the first modification.
In a special embodiment the wavefront modifier operates as an aberration compensator, correcting an undesired aberration in a radiation beam. When the modifier introduces a decentred aberration, the amount of the aberration may be controlled by the output signal of an aberration detector for measuring the aberration in the radiation beam and the amount of decentring may be controlled by the output of a position detector for measuring the displacement of the radiation beam. Alternatively, the amount of the decentred first aberration is controlled by a combination of the output signals of the aberration detector and the position detector, the magnitude of the decentring being fixed, and the sign of the decentring is controlled by the sign of the output signal of the position detector. When the modifier introduces a centred first and second aberration, the amount of the first aberration may be controlled by the output signal of the aberration detector and the amount of the second aberration by the output signal of the position detector.
The aberration detector may be a tilt detector for detecting tilt of the record carrier, and the aberration compensator introduces coma as a first aberration in the radiation beam, which compensates the coma caused by the tilt. The astigmatism caused by the off-centre objective system is preferably compensated by astigmatism introduced as a second aberration by the aberration compensator.
In a preferred embodiment, the wavefront modifier introduces coma and astigmatism by means of two similar, transversely displaced electrode configuration. Each of these structures introduces coma in the radiation beam. Since the coma of each configuration is off-centre, it can be described as a combination of centred coma and astigmatism. If the objective system is off-centre in one direction, one set of electrode structures will be energised; when the objective system is offset in the opposite direction, the other set of electrodes will be energised.
In a special embodiment the wavefront modifier comprises a series of strip electrodes in a symmetrical arrangement. The strips may have a curved shape. Any combination of aberrations may be obtained by setting each of the strips at a specific voltage. In a preferred embodiment the voltages are formed by a series arrangement of resistors having taps the strip electrodes being connected to the taps. The desired voltage distribution over the strips can be achieved by setting the voltage at the end taps and at least one intermediate tap. The electrode structure is preferably etched in a transparent conductive layer. The same conductive layer may be used to form the series arrangement of resistors.
The first and second wavefront modifications are preferably orthogonal over the cross-section of the radiation beam to ensure that the first and second modification do not affect each other, thereby improving the independence of the control of the modifications.
A further aspect of the invention relates to a device for scanning an optical record carrier having an information layer, comprising a radiation source for generating a radiation beam, an objective system for converging the radiation beam to a focus on the information layer, and a detection system for intercepting radiation from the record carrier, characterized in that an optical wavefront modifier according to any of the preceding claims is arranged in the optical path between the radiation source and the detection system.